A. Field of the Invention
The invention generally concerns a method for producing hydrogen gas (H2) and calcium carbonate (CaCO3) from formaldehyde. In particular, an aqueous basic composition containing formaldehyde and a transition metal complex having a coordination bond between a transition metal and a leaving group can produce hydrogen gas and formate or a salt thereof from the formaldehyde. The formate or salt thereof can then be used as a carbon source for the production of CaCO3 and formaldehyde, the latter of which can be recycled to produce more hydrogen gas and CaCO3.
B. Description of Related Art
There is increasing global demand for hydrogen gas. Conventional technology produces hydrogen from steam reforming of methane as shown in the equations (1) and (2) below. The major source of the methane is from natural gas:CH4+H2O→CO+3H2  (1)CO+H2O→CO2+H2  (2)Due to the depletion of fossil fuels, there is a necessity to find an alternative feedstock to meet the growing demand for hydrogen production globally.
As for calcium carbonate (CaCO3), it finds many uses across a variety of industries. Some examples include its use in the (1) construction industry as a building material or in the production of cement, (2) oil industry as an additive to drilling fluids as a formation-bridging and filtercake-sealing agent, (3) food industry as a raw materials for refining sugar from sugar beet, (4) adhesive industry as an ingredient in adhesives, sealants, and decorating fillers, (5) paining industry as a paint extender, (5) medical industry as a calcium supplement or gastric antacid, or (6) pharmaceutical industry as a filler for tablets and other pharmaceutical dosage forms.
A variety of processes for producing hydrogen gas and calcium carbonate have been proposed. With respect to hydrogen gas production, some processes range from water-splitting, thermal dehydrogenation of formic acid, catalytic dehydrogenation of small organic molecules, or thermal dehydrogenation of amino-boranes and the like. Dehydrogenation of small organic molecules such as formic acid, methanol and formaldehyde has been attempted. Dehydrogenation of formic acid into hydrogen and carbon dioxide suffers in that the reaction is inefficient as formic acid has a low hydrogen content (about 4.4 wt. %). Further, the production of carbon dioxide can be problematic.
As for methanol, while it has a high hydrogen content (12.5 wt. %), the dehydrogenation process suffers in that the catalysts used to promote the dehydrogenation are sensitive to air and easily decompose. Further, methanol reforming is conducted at high temperatures (200° C.) and pressures (>25 bar), thereby limiting the scalability of the process.
With respect to formaldehyde, while there have been attempts to use formaldehyde in hydrogen production processes, the processes can require additional materials and/or use high temperatures, thereby making the processes inefficient and difficult to scale-up for mass hydrogen gas production. By way of example, International Application Publication No. WO 2014/204200 to Yoon et al. describes the dehydrogenation of methanol in the presence of formaldehyde using a palladium oxide on titanium dioxide photocatalyst to produce hydrogen. International Application Publication No. WO 2015/003680 to Prechtl et al. describes thermal process for generating hydrogen by heating formaldehyde-containing wastewater at 95° C. in the presence of a catalyst having a dimeric form of ruthenium with aromatic hydrocarbon ligands. Wang et al. in “Novel microbial synthesis of Cu doped LaCoO3 photocatalyst and its high efficient hydrogen production from formaldehyde solution under visible light irradiation,” Fuel, 2015, Vol. 140, pp. 267-274 describes preparation of a copper doped LaCoO3 using microorganisms. Kapoor et al. in “Kinetics of Hydrogen Formation from Formaldehyde in Basic Aqueous Solutions,” Journal of Physical Chemistry, 1995, Vol. 99 describes the kinetics of thermal generation of hydrogen from solutions of formaldehyde in the form of HCHO, with an increase in hydrogen production observed by an increase in reaction temperatures. Notably, Kapoor et al. also explains that hydrogen production is from HCHO and trioxane and is not from para-formaldehyde.
In addition to the inefficiencies of the systems discussed above, photocatalytic attempts to produce hydrogen from aqueous formaldehyde solutions have typically relied on water splitting to generate electron holes that oxidize the formaldehyde to formic acid. Subsequent photo-oxidation of the formic acid produces hydrogen and carbon dioxide through a multi-step process shown in equations (3) through (9) below.

Regarding calcium carbonate production, the majority of calcium carbonate is extracted from the earth through mining or quarrying operations. For chemical synthesis, the conventional process includes mixing water with calcium oxide to produce a calcium hydroxide solution. Carbon dioxide is then passed through the solution to precipitate calcium carbonate.